Baffling Canon EOS autofocus puzzle

StewartR

StewartR

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I's really welcome any insight that can be offered into this problem that one of our customers has - or thinks he has.

He's hired a Canon 10-22mm wide-angle to go on his EOS 650D. His observation was that, when he's allowing the camera to choose which AF points to use, it only ever seems to choose points on the left hand side of the frame. So why is that?
  1. normal behaviour?
  2. camera problem?
  3. lens problem?
  4. user error?

I suggested he tried forcing the camera to use just one AF point, and he confirmed that they all work.

I suggested framing a picture so that the left side of the image was contrasty and the right was plain (e.g. a blank wall), and the camera correctly chose AF points on the left. Then I suggested he turn the camera upside down and try it again, and this time it correctly chose AF points on the left side of the image, which was now the right side of the camera.

So it will use the AF points on the right if it's told to, and it will use them if there's really nothing with any contrast on the left. But for "normal" shooting (whatever my customer defines as "normal"!) he says it always seems to choose points on the left.

Any ideas? Is that normal behaviour? Is there any reasonably straightforward set of circumstances in which one would expect it?
 
May sound daft but try taking a picture with the camera upside down. This will differentiate between the camera using only the left AF points or it is something in the outside world.

Next thought - factory reset.
 
Full auto with the nearest items always on the left? So it always tries to focus on nearest item?
 
Oggy said:
May sound daft but try taking a picture with the camera upside down. This will differentiate between the camera using only the left AF points or it is something in the outside world.
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I thought of that:
StewartR said:
I suggested framing a picture so that the left side of the image was contrasty and the right was plain (e.g. a blank wall), and the camera correctly chose AF points on the left. Then I suggested he turn the camera upside down and try it again, and this time it correctly chose AF points on the left side of the image, which was now the right side of the camera.
Click to expand...
 
MatBin said:
Full auto with the nearest items always on the left? So it always tries to focus on nearest item?
Click to expand...
I'm afraid I don't have any information to answer those questions. I don't know what was typically in the frame when the customer said it nearly always used the left AF points. And I don't know how Canon cameras decide which objects to focus on, if there are objects at different distances.
 
StewartR said:
.....And I don't know how Canon cameras decide which objects to focus on, if there are objects at different distances.
Click to expand...

My understanding is that it will focus on the nearest subject that falls within an active AF point. All other AF points that lie within the same plane (+ DoF) will also illuminate.

Bob

Edit.....this from a post in the Digital Journalist (Chuck Westfall Q&A column)
When multiple focusing points are illuminated in the combination of One-Shot AF with automatic focusing point selection, it means that all of them are within the same depth of focus as each other. In such cases, the camera usually selects the closest focusing point with reliable data. In the case of AI Servo AF and automatic focusing point selection, AF frames are not illuminated while the subject is being tracked. In cases where the subject is large enough that multiple focusing points are within the same depth of focus, a similar rule for focusing point selection is applied. In both cases, data from only one focusing point is actually selected and used by the camera.
 
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4. :D

And as Bob explained, auto AF point selection goes to the nearest subject area covered by a point, though if there is a high contrast area behind that also falls under an AF point it can get drawn to that.

Edit: AFAIK all AF systems work like this, and is the underlying reason why auto AF point selection is useless.
 
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Canon Bob said:
My understanding is that it will focus on the nearest subject that falls within an active AF point. All other AF points that lie within the same plane (+ DoF) will also illuminate.
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HoppyUK said:
And as Bob explained, auto AF point selection goes to the nearest subject area covered by a point, though if there is a high contrast area behind that also falls under an AF point it can get drawn to that.
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That makes sense. Thanks guys. I was hoping one of you would show up.
 
HoppyUK said:
4. :D
...
AFAIK all AF systems work like this, and is the underlying reason why auto AF point selection is useless.
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Tell me about it. I spent 15 minutes on the phone to the guy trying to explain to him that regardless of whether or not auto AF point selection was working properly, he'd be better off deciding himself what he wanted to be in focus. He didn't seem to understand. That's mainly why I included option 4 in the list.
 
Well, if it is any help I have the same lens and I can tell you that I don't have that problem. So it is not normal behaviour.
 
Meeplemaiden said:
Well, if it is any help I have the same lens and I can tell you that I don't have that problem. So it is not normal behaviour.
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Agreed. I have about 15 or 16 of them and I've never come across this before.

But then how many people (a) use auto AF point selection, and (b) notice which points are actually selected? I didn't want to rule out the possibility that it is normal behaviour, but most people just don't notice it. It occurred to me that Canon's AF algorithms might have some subtle bias towards one side, but it appears probably not.
 
Just a thought.....
If he's using the LCD and not the viewfinder then he might be touching the left side of the LCD and selecting a left side focus point unwittingly with the 650D's "Flexizone" facility?

Bob
 
That lens is almost a fish eye lens, therefore what is in focus on either side would be the closest and what the camera would auto focus on first. Therefore he would need to come out of auto AF and select a narrower focal point manually. My Nikon camera have a 9 -21-51 dynamic area so presumably Canon have something similar. If I used that lens in auto focus it would pick out the extreme left/right focal point first presumably
 
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